Tetanus toxin can bind to certain gangliosides present in neuronal cell membranes, however it is not clear if these gangliosides constitute a functional receptor. The ability of tetanus toxin to bind ganglioside is retained by Fragment C, a non-toxic fragment of tetanus toxin consisting of the COOH-terminal third of the molecule. Our studies are directed at understanding the importance of gangliosides as a receptor for tetanus toxin. In previous work, Fragment C was cloned into an expression vector and demonstrated to be indistinguishable from tetanus toxin-derived Fragment C. We have constructed additional clones of Fragment C with sequential deletions from the NH2 terminal in order to identify the sequence essential for ganglioside binding. The proteins produced from these clones were highly susceptible to proteolysis. One apparent degradation product of approximately 25 kDa was partially purified based upon its ability to bind to ganglioside. Experiments with anti-peptide antibodies demonstrated that this peptide is derived from the COOH-terminal of Fragment C. This peptide bound to neuronal cells similarly to Fragment C and intact tetanus toxin. Our results to date indicate that tetanus toxin binds to ganglioside through a site in the C-terminal region of Fragment C.